This paper describes a measurement method for three-dimensional (3D) slit width deviations of long precision slot dies, which are essential for process control in manufacturing. A sensor unit consisting of two laser probes with their measurement axes aligned along the same Z-directional line but with opposite measurement directions, is placed between the two parts of the slot die to scan the two opposing surfaces of the parts along the X- and Y-axes. The variation of the sum of the laser probe outputs, which shows the deviation of the distance between the two surfaces, corresponds to the deviation of the slit width in the Z-direction. The 3D slit width deviations can be obtained accurately through scanning the entire surface in the X Y plane. In addition, the surface flatness of the parts can also be measured accurately by adding one more probe. Measurement experiments have been conducted on a precision grinding machine. The measurement results have indicated that the 3D slit width deviations and flatness can be measured with a repeatability error of less than 1 micron, which meets the requirement for quality control of slot dies.